The present invention relates to an optical device suitable for optically recording an information signal on an information carrier or optically reproducing an information signal on the information carrier by means of a ray beam emitted from a ray beam source.
In optical recording or reproduction of information into and out of an information carrier (photo-disc), the ray beam from a ray source is applied to a lens system (objective lens) adapted to converge the ray beam and optimize the same for the recording or reproduction within the diffraction limit of the objective lens. The dimensions of the ray beam spot is determined by the number NA of apertures of the objective lens and the wavelength .lambda. of the incident ray beam. More specifically, the dimensions of the ray beam spot is approximated by .lambda./NA. For instance, assuming that the ray source is a semiconductor laser beam source of a wave length .lambda. of 0.8 .mu.m and that the number NA of apertures of the objective lens is 0.6, the dimension of the beam spot is calculated to be about 1.3 .mu.m.
The information signal which is to be recorded or reproduced by such a fine beam spot has the form of a pit having a breadth of 0.4 to 0.8 .mu.m and a length of 0.5 to 2 .mu.m. In order to record or reproduce such a pit-like signal, it is necessary to employ a focussing servo system for controlling the focal point of the beam spot, as well as a tracking servo system for correctly applying the ray beam to the information signal track. In some cases, it is necessary to adopt a time base correction (TBS servo) which is adapted to shift the beam spot in the direction of the track to compensate for any fluctuation of signal frequency in relation to time attributable to a fluctuation in the rotation speed of the disk or the like.
These servo systems should have extremely high precision. For instance, the focussing servo system is required to have a dynamic range in the order of several hundreds of microns at an allowance of .+-.1 .mu.m, while the tracking servo system has to have a dynamic range of .+-.100 .mu.m or greater at an allowance of .+-.0.1 .mu.m. The TBS servo system requires a dynamic range substantially equivalent to that of the tracking servo system.
For realizing such servo system of high precision, it is necessary to use a small-sized and light-weight optical pickup capable of moving two-dimensionally or three dimensionally. The construction and operation of a typical optical pickup will be explained hereinunder with reference to the accompanying drawings.